The present invention relates to a method and apparatus that initiates a crowbar protection mode in a shunt regulator in response to variable time and current criteria. More specifically, the present invention uses a piecewise linear approximation method to dynamically activate a crowbar mode by monitoring time and intensity of current conduction in the shunt regulator.
Overcharging is an issue that must be addressed in a battery protection circuits. Lithium based batteries, including Lithium-Ion batteries and Lithium-Polymer batteries tend to be sensitive to excessive voltages. Without a suitable safety circuit overcharging may compromise the batteries reliability.
For improved reliability, chargers and battery packs include devices that bypass the battery charging current when charging becomes excessive. Such devices detect excessive charging current and reroute the charging current through a shunt circuit. One such device is a xe2x80x9cZener-fusexe2x80x9d circuit as shown in FIG. 1.
The xe2x80x9cZener-fusexe2x80x9d circuit shown in FIG. 1 includes a power supply/charger (102), a fuse (108), a zener diode (106), and a battery cell (104). The power supply/charger (102) includes a power terminal (PWR) that is connected to node N10, and a ground terminal (GND) that is connected to node N12. The fuse (108) is series connected between node N10 and node N11. The zener diode (106) has a cathode that is connected to node N11 and an anode that is connected to node N12. A battery cell (104) has a positive terminal connected to node N11 and a negative terminal connected to node N12. Node N12 is a circuit ground potential.
The power supply charger (102) is arranged to provide a charging current to the battery cell (104) through the fuse (108). The zener diode (106) is connected in parallel with the battery cell. In this circuit, the zener diode (106) begins conducting in the reverse-biased, or xe2x80x9cavalanchexe2x80x9d, mode when the voltage from the power supply/charger (102) exceeds the normal charging voltage of the battery cell (104). Once the zener diode (106) is in the avalanche mode, the zener diode acts as a short circuit relative to the power supply/charger (102). The avalanche condition of the zener causes the current to increase rapidly which causes the fuse (108) to clear, isolating the battery cell (104) from the power supply/charger (102).
The circuit shown in FIG. 1 does not produce a perfect short circuit condition when the zener diode (106) conducts in the avalanche mode. Instead, a voltage develops across the zener diode (106) causing it to dissipate power. As the zener diode (106) begins conducting higher currents, the zener diode (106) rapidly generates heat, creating a thermal race condition between the fuse (108) and the zener diode (106). In order to safely clear the fuse (108), the zener diode (106) must experience thermal degradation at a slower rate than the fuse (108). In order to ensure that the fuse (108) clears before the zener diode (106) reaches a catastrophic temperature (a temperature that causes the zener diode to fail), a zener diode (106) with a high power rating must be employed. High power zener diodes are often big, bulky, and expensive.
In accordance with the present invention, an apparatus and method provides for enhanced crowbar protection in a shunt regulator. The crowbar protection is dynamically tuned to maximize safe power performance in the shunt regulator without interruption from over-current protection. In one example of the present invention, an improved crowbar protection circuit has a continuously variable threshold that maximizes the safe operating range of the shunt regulator. In another example of the present invention, an improved crowbar protection circuit has a piece-wise approximation of a thermal crowbar protection profile such that multiple threshold points are used to selectively activate crowbar protection based on a given current conduction level and an associated transient time for the given current conduction level.
Briefly stated, the present invention relates to a method and apparatus provide for improved crowbar protection in a shunt regulator circuit. The shunt regulator includes a shunt device that generates thermal energy during conduction. An over-temperature protection circuit may be combined with a fast-crowbar protection circuit such that maximum protection from damaging thermal energy is provided to the shunt device. Thermal energy develops in the shunt device at a rate that is faster than an associated time for heat to physically transfer to a thermal sensor. The fast-crowbar protection circuit estimates the thermal energy in the shunt device based upon an integration method. By integrating a measured power over time the rise in temperature can be estimated such that the crowbar protection is enabled before the thermal energy can damage the shunt device. The integration method can be approximated using a piece-wise linear approximation such that the estimation circuitry can be simplified. A series of comparators and timing/delay circuits are employed to measure a current level in the shunt device over a given duration. The timing/delay circuits have memory such that heat build up and heat dissipation are modeled. In one example, a capacitor circuit is used to generate a time constant for the timing/delay circuit. The capacitor circuits associated charging and discharging times are different such that thermal memory is modeled.
According to a feature of the invention, an apparatus is directed to estimating a temperature in a shunt circuit that includes a transistor having an associated shunt current, an associated ambient temperature, and an associated heat dissipation factor. The apparatus includes a measurement circuit that is arranged to produce a measurement signal that is associated with the shunt current. A temperature measurement circuit may optionally be arranged to produce a temperature measurement signal that corresponds to the ambient temperature. An integration circuit is arranged to produce an integration signal in response to the measurement signal such that the integration signal corresponds to an integral of the measurement signal over a time interval. The integration signal corresponds to the rise in temperature in the shunt circuit such that the temperature of the shunt circuit is estimated using the integration signal. When the optional temperature measurement circuit is used, the temperature measurement signal may be used in conjunction with the integration signal to estimate the temperature in the shunt circuit.
According to another feature of the invention, an apparatus is directed to producing a detection signal that indicates that an estimated temperature has exceeded a safety temperature in a circuit that includes a transistor circuit with an associated operating current. The apparatus includes a means for measuring current that is arranged to measure the operating current of the transistor and produce a measured operating current. Optionally, a means for measuring temperature is arranged to measure the ambient temperature at an initial time and produce a measured ambient temperature that is associated with the transistor. A means for integrating is arranged to integrate the measured operating current over a time interval to produce an integration signal. A means for estimating is arranged to provide the estimated temperature in response to the integration signal. The integration signal corresponds to a rise in the ambient temperature of the transistor circuit. A means for comparing is arranged to compare the estimated temperature to the safety temperature and produce a fast detection signal. The fast detection signal indicates that the estimated temperature has exceeded the safety temperature. The detection signal is responsive to the fast detection signal.
According to yet another feature of the invention, an apparatus is directed to estimating a temperature in a shunt circuit that includes a transistor having an associated shunt current, an associated ambient temperature, and an associated heat dissipation factor. The apparatus includes a measurement circuit that is arranged to produce a measurement signal that is associated with the shunt current. A bank of N comparator circuits is also included. Each of the comparator circuits produces a corresponding detection signal in response to a comparison between the measurement signal and a corresponding reference signal. Each detection signal indicates that the shunt current has exceeded a corresponding current threshold level that is determined by the corresponding reference signal. A bank of N timing/delay circuits is also included. Each of the bank of N timing/delay circuits produces a corresponding timeout signal when a corresponding one of the detection signals has persisted for a corresponding delay time interval. A combination circuit combines the N timeout signals to produce a fast detection signal. The fast detection signal indicates that the shunt current has exceeded at least one of the current threshold levels for the corresponding delay time interval.
According to still another feature of the invention, a method is directed to estimating a temperature in a shunt device that has an ambient temperature and an operating current level. The method includes sensing the operating current level of the shunt device to produce a sense signal, integrating the sense signal over a time interval from the initial time to a subsequent time to produce an estimated temperature rise signal, and estimating the temperature in the shunt device in response to the estimated temperature rise signal. Also, by comparing the estimated temperature rise signal to a reference signal that is related to the ambient temperature, a fast detection signal is produced that indicates the estimated temperature rise signal has exceeded a safety margin for the shunt device.
According to a further feature of the invention, the integration of the sense signal may be approximated using a piece-wise linear approximation. The piece-wise linear approximation is implemented by comparing the sense signal to a first reference signal and producing a first detection signal that corresponds to a first operating current level of the shunt device, and, comparing the sense signal to a second reference signal to produce a second detection signal that corresponds to a second operating current level that is different from the first operating current level. A first capacitive circuit is charged at a first charge rate in response to the first detection signal when the sense signal indicates that the operating current level is substantially greater than the first operating current level. The first capacitive circuit has a first potential associated therewith. A second capacitive circuit is charged at a second charge rate in response to the second detection signal when the sense signal indicates that the operating current level has exceeded the second operating current level. The second capacitive circuit has a second potential associated therewith. The first capacitive circuit is discharged at a first discharge rate in response to the first detection signal when the sense signal indicates that the operating current level is substantially less than the first operating current level. The second capacitive circuit is discharged at a second discharge rate in response to the second detection signal when the sense signal indicates that the operating current level is substantially less than the second operating current level. Detecting that the first potential has exceeded a first reference signal produces a first detection signal. Detecting that the second potential has exceeded a second reference signal produces a second detection signal. The method determines that the estimated temperature rise signal has exceeded a safety margin when indicated by at least one of the first detection signal and the second detection signal.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are embodiments of the invention briefly summarized below, to the following detail description of presently preferred, and to the appended claims.